A static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This static magnetic field is referred to as the Bo field. It is commonly known that increasing the homogeneity of the Bo field used for performing an MRI scan increases the quality of the diagnostic images, which benefits physicians using an MRI image to diagnose a patient.
Magnetic Resonance Imaging (MRI) utilizes very high magnetic field uniformity on the order of 10 parts-per-million (ppm) over the field-of-view (FOV), typically about 50 cm diameter sphere. A variety of shimming techniques are used to compensate for the magnetic field non-uniformity that occurs due to manufacturing tolerances of the magnet, external magnetic environment, or field disturbances introduced by the investigated subject.
For instance, the major manufacturing and environmental magnetic field disturbances are addressed by either passive shims, or superconducting shims, or both. Passive shims are typically precisely-positioned pieces of magnetic material such as iron. The strong active superconducting shims are located inside the cryostat. Typically, the shims are not re-adjusted for a particular patient. Resistive shims are used for compensation of relatively small field disturbances introduced by the investigated subject. In MRI scanners, resistive shims are located inside the gradient coil assembly. Both types of the active electric shims are used in Nuclear Magnetic Resonance (NMR) and Fourier Transform Ion Cyclotron Resonance (FT-ICR) systems.
Recent advances in MRI technology have sought to include increased field strength of the scanners, increased aperture MRI systems and more powerful, higher linearity gradient coils. In these systems, radial space available to active shims is restricted. This is a serious constraint for resistive shims. At the same time, applications such as diffusion and functional imaging in brain require versatile resistive shims that are capable of compensating yet higher-order spherical harmonics. orthogonally
The magnetic field inside the magnet in a source-free region is described by the Laplace equation ∇2 Bz=0. Solution of this equation in spherical coordinates is:Bz(r,θ,φ)=Σn=0∞ΣM=0NANMrNPNM(cos θ)eiMφ,  (EQUATION 1)
where PNM=PNM(cos θ) are associated Legendre polynomials of the degree N and order M; index N runs from zero to infinity; index M changes from zero to N;
            cos      ⁢                          ⁢      φ        =          z      r        ,                    cos        ⁢                                  ⁢        θ            =              x                  r          ⁢                                          ⁢          sin          ⁢                                          ⁢          φ                      ;                  sin        ⁢                                  ⁢        θ            =              y                  r          ⁢                                          ⁢          sin          ⁢                                          ⁢          φ                      ,and x, y and z are Cartesian coordinates of a point, r=(x2+y2+z2)1/2.
The dimensional product ANMrN PNM (cos θ)eiMφ in Equation 1 represents orthogonal cosine and sine N, M spherical harmonics of the magnetic field; these harmonics are often noted as the (N, M) harmonics, or terms. The field harmonics with M=0 are often called axial harmonics; these harmonics do not depend on the angle φ. Harmonics with M>0 are called radial, or tesseral harmonics; these harmonics do depend on the angle φ.
Those skilled in the art recognize two approaches as to design of the electric shims:
1st Approach:
Orthogonal shim approach. Each shim is designed to generate a certain field harmonic while other harmonics are relatively small or zero. The orthogonal shims are distinguished by application (axial, M=0, and radial shims, M>0) and degree (N=0, 1, 2, 3, . . . ).
The first order components of the spherical harmonics are typically shimmed using the magnetic field gradient coils and the higher order terms are shimmed using dedicated shimming coils. The magnetic field gradient coils create gradients in the magnetic field in order to spatially encode the radio frequency signals that the nuclei emit during MRI.
2nd Approach:
Matrix shims. MRI systems include multiple shims each of which may simultaneously generate a variety of odd, even, axial and radial terms.
Several approaches have been proposed to make more compact and more efficient resistive shims. Most are dedicated to so-called matrix shims, see, for example. The matrix shims occupy a smaller radial space at a penalty of complicated circuitry, a significant number of external drivers (typically more than twice as many drivers versus traditional orthogonal shims), along with complicated software to control the drivers.
A side-by-side location of the resistive shims has also been proposed to reduce the volume required to contain the shimming coils by limiting the azimuthal span of the individual saddle coils. This allows shim coils of the same degree and order to be combined together in the same radial layer. The side-by-side located shims, however, may exhibit known disadvantages including reduced shim strength due to smaller angle of the arc and appearance of the unwanted terms.
A need exists to address the multiple trade-offs, including shim performance (e.g. strength, purity, interaction with other components of the scanner), spatial constraints, ease of use, shim wiring complexity, the number and sophistication of external drivers, sensitivity to positional tolerances and other issues. The proposed invention will further improve shimming capability by proposing nested saddle coils of different order and/or degree inside each other.